In processes of obtaining a metal container such as a seamless can, a cup is formed from a flat sheet material (blank) according to deep-drawing and the obtained cup is redrawn or subjected to redrawing and ironing to obtain a seamless can. The deep-drawing is realized in such a way that a draw punch squeezes a blank into a draw die in a state where the blank is clamped between a pressure pad (also referred to as a blank holder, a draw pad or the like) and the upper surface of the draw die. However, in this case, when the deep-drawing progresses, wrinkles are occurred in the blank and the wrinkles may remain on the side walls of a formed cup unless the wrinkles are suppressed by the pressure pad and the draw die. If wrinkle pressing pressure is increased to suppress the occurrence of wrinkles, large tension acts on the cup during the deep-drawing and the bottom of the cup may collapse easily. Such a phenomenon becomes remarkable as the drawing ratio increases. Although the drawing ratio is naturally limited depending on materials, various attempts have been proposed to suppress the occurrence of wrinkles and the collapse of the bottom and to improve the drawing ratio by modifying the pressure pad in order to perform deep-drawing satisfactorily.
Although a pressing surface of a pressure pad is generally formed as a flat surface, for example, a technique of forming a concentric ring-shaped groove on the pressing surface to form the pressing surface into a concavo-convex shape, forming small wrinkles intentionally in the ring-shaped groove in a drawing process to generate appropriate tension in a blank member by allowing the wrinkles to be hooked on a transitional portion of the concavo-convex shape to thereby prevent the occurrence of large wrinkles and pinching has been proposed (Patent document 1).
Moreover, a technique of forming a concave portion in a pressure pad to form the pressing surface of the concave portion as a tapered surface which becomes deeper as the surface advances toward an outer circumference has been proposed (Patent document 2).
However, both techniques are not satisfactory enough to suppress wrinkles occurred or to prevent the collapse of the bottom in the blank at the initial stage of the forming when a thin base metal sheet is deep-drawn with a high drawing ratio.
Moreover, seamless cans are also manufactured using a resin-coated metal sheet in which one or both surfaces of a metal base is coated with a resin such as a polyester resin. However, in the case of positive pressure cans, since the metal sheet is thin and the blank diameter is small, such cans may be generally obtained through deep-drawing one round of redrawing and a plurality of rounds of ironing and the number of processes required for obtaining the diameter of a final can is small.
On the other hand, in the case of negative pressure cans, since the metal sheet is thick and the blank diameter is large as compared to positive pressure cans, the number of redrawing processes required for obtaining the diameter of a final can increase and a multi-process press machine is used. However, the productivity is poor and the facility and die costs are high, and die exchange require a lot of time.
Moreover, although the number of processes can be reduced when the drawing ratio in deep-drawing and redrawing processes are increased, if the drawing ratio is simply increased forming defects such as cup side wall wrinkles or the collapse of the bottom occur.
Further, in recent years, there is a demand for decreasing the thickness of a base sheet from the perspective of weight reduction, and as a result, forming defects such as cup side wall wrinkles or the collapse of the bottom are likely to occur during deep-drawing. Thus, it is desirable to prevent such forming defects when a thin metal sheet such as tin-free steel is deep-drawn.